After releasing nav-menu-template_14896.php in build 14896, a new version was made available (also called 14896) with a bunch of differences.
Particularly function _wp_menu_item_classes_by_context( &$menu_items = array() )
disappeared.
The problem seems to disappear/reappear. As I am going nuts trying to track down with diffs what’s going on to that file, I have to leave this to the experts. Do the latest-nightly-updates.zip files constantly change content?